Recently, the frequency characteristics of a sound signal have even reached a high bandwidth. Although the reproduction band of conventional CDs was 22.05 kHz (sampling frequency 44.1 kHz), in the current high-resolution audio system, the reproduction band of 48 kHz (sampling frequency 96 kHz) or 96 kHz (sampling frequency 192 kHz) is also becoming common. On the other hand, since the performance of sound pickup devices of the previous generation was inferior to the performance of sound pickup devices of recent years, high frequency components are not recorded in music contents of the previous generation. Additionally, some music contents lack high frequency components due to compression coding by compression methods of the previous generation.
A signal processing technology of recovering high frequency components for such music contents is known.
For example, Patent Literature 1 discloses a technology of compensating high frequency harmonic components by copying harmonic components remaining in a middle frequency band to high frequencies. Additionally, Patent Literature 2 discloses a technology of compensating high frequency harmonic components by performing folding of low frequency components to high frequency components, when a signal downsampled at a ½ rate is upsampled at twice the rate.